1. Field of the Invention
The present invention relates to an optical scanning apparatus and an image forming apparatus using the same, and is suitable for an image forming apparatus, such as a laser beam printer, a digital copying machine, or a multi-function printer, employing an electrophotography process.
2. Description of the Related Art
Hitherto, in an optical scanning apparatus of a laser beam printer (LBP) or of a digital copying machine, a light beam optically modulated and emitted from light source unit in accordance with an image signal is periodically deflected by an optical deflector (deflecting unit) constituted by a rotary polygon mirror (polygon mirror). Then, the deflected light beam is converged into a spot-like shape on a surface of a photosensitive recording medium (photosensitive drum) by an imaging optical system having an fθ characteristic, and the surface thereof has been optically scanned, to thereby effect image recording.
In the optical scanning apparatus of this type, adjustment of a focus position on a surface to be scanned is carried out by moving along an optical axis at least a part of lenses that constitute an incident optical system. The various optical scanning apparatus described above have conventionally been proposed (refer to Japanese Application Laid-open No. H02-181712 and Japanese Application Laid-open No. H08-304724).
Providing a lens for adjusting the focus position to an incident optical system includes a problem in which, there is given a matter that the scanning optical system which aims to attain high resolution, that is, an optical system for forming a small spot on the surface to be scanned, has a small focal range, i.e., a small depth of focus, at which a satisfactory spot may be obtained.
As factors of deviations of the focus position, the following two factors are given:    (1) initial focus deviation due to the precision in positioning of parts at the time of assembling of the scanning optical system, or surface accuracy of optical components; and    (2) focus deviation due to positional displacements of structural parts accompanied by temperature change or change in characteristics of the optical components. If the depth of focus is small, the focus deviation causes due to the above-mentioned factors (1) and (2). As a result, the surface to be scanned goes out of the depth of focus.
To address the problems described above, for example, the construction disclosed in the fifth embodiment (FIGS. 7 and 8) of Japanese Patent Application Laid-open No. H02-181712 is illustrated in FIGS. 8 and 9.
Japanese Patent Application Laid-open No. H02-181712 points out that, as a factor of causing the focus deviation accompanied by the temperature change, in a case where a semiconductor laser is used for the light source unit, the wavelength fluctuates depending on temperature, and chromatic aberration of the lens occurs, resulting in the deviation of the focus position. Besides, as factors of causing an initial focus deviation, pointed out are a positional displacement between the surface to be scanned (photosensitive drum) and the optical scanning apparatus, and processing precision of the optical components (surface accuracy).
As a counter measure against those problems, Japanese Patent Application Laid-open No. H02-181712 discloses a conversion optical system (collimator optical system) La constituted by three-lens/two-unit system, including in order from the light source side a cemented lens 2 having a negative power and formed by two pieces of lenses including a positive lens and a negative lens; and an aspherical lens 4 having a positive power. By configuring the third lens 4 as an aspherical lens, there is constructed an excellent collimator optical system La capable of further suppressing the spherical aberration,
The collimator optical system, which is generally constructed by a plurality of lenses, is typically constructed by lens units having positive and negative powers in view of correcting the spherical aberration. In Japanese Patent Application Laid-open No. H02-181712, the wavelength of a laser light oscillated by a laser light source (light source unit) 1 shifts due to the change in temperature, and the chromatic aberration of the lens causes the focus deviation. For this reason, the cemented lens 2 is rendered to have a function of over-correcting achromatization, to thereby cancel the chromatic aberration caused in the imaging optical system 6.
In addition, the cemented lens 2 on the light source unit side has a construction so as to be adjustable in an optical axis direction, and the positional displacement between the surface to be scanned (photosensitive drum) 7 and the optical scanning apparatus, and uniform focus deviation of a scanning lens 6 from a designed value caused by the processing error are corrected, to thereby align the focus position to a center of the depth of focus.
Note that, a ratio between the power of the cemented lens 2 as the adjustment lens and the power of the entire system of the three-lens/two-unit system is as small as −0.2, and hence the adjustment sensitivity is low, and which makes the adjustment easier.
On the other hand, Japanese Patent Application Laid-open No. H08-304724 proposes a method, which achieves the reduction of the sensitivity for the focus adjustment by constructing the collimator optical system by one collimator lens and one adjustment lens, and by moving the adjustment lens in the optical axis direction.
FIGS. 9 to 11 represent a schematic view of a main portion of the optical scanning apparatus, and cross-sections of the lenses of the optical system, respectively, which are disclosed as FIGS. 1, 1A, and 1B of the first embodiment in Japanese Application Laid-open No. H08-304724.
In Japanese Patent Application Laid-open No. H08-304724, the light source unit 1 is disposed at a position closer by 1 mm to the collimator lens from the focal length of the collimator lens 2. Further, the light beam, which has entered into the collimator lens 2, is converted into a diverged light beam after passage therethrough, and is converted into parallel light beams by a spherical lens 15 having convex (positive) power, which is arranged in the optical deflector 5 side of the collimator lens 2. The converted parallel light beams is imaged into a spot on the surface to be scanned (image plane) 7 via the cylindrical lens 4 and the optical deflector 5 by the imaging lens 6. The focal length f of the collimator lens 2 is 8 mm, the focal length of the spherical lens 15 is 172 mm, an interval between the collimator lens 2 and the spherical lens 15 is 100 mm, and the focal length of the imaging lens 6 is 480 mm.
In the construction, a movement amount of the spherical lens 15 becomes 0.64 mm so as to deal with the focus deviation of 5 mm on the surface to be scanned 7. The movement amount of the collimator lens 2, in a case where the spherical lens 15 does not exist, was 0.0014 mm, and hence it results in reducing the adjustment sensitivity by 1/460.
The above-mentioned Japanese Application Laid-open No. H02-181712 and Japanese Application Laid-open No. H08-304724 do not disclose at all the adjustment sensitivity within the sub-scanning section if a lens which is adjustable in the optical axis direction is caused to move.
Generally, if the spherical lens is caused to move, the focus position on the image plane deviates in both the main scanning direction and the sub-scanning direction, thereby being not able to perform focus adjustments in the main scanning direction and the sub-scanning direction, individually, which being a problem.
The amount of focus deviation in the sub-scanning direction which occurred when the focus adjustment in the main scanning direction was carried out, can be adjusted in the sub-scanning direction by adjusting the cylindrical lens. However, the focus adjustment in the main scanning direction causes the focus deviation in the sub-scanning direction, resulting in increase in adjustment amount of the cylindrical lens. This makes an adjustment range of the cylindrical lens longer, and requires the cylindrical lens securing an accurate posture during the adjustment throughout a long distance. As a result, a seating surface is required to be processed with high precision.
Japanese Patent Application Laid-open No. H08-304724 also discloses a case where a cylindrical lens having a power only in the main scanning direction is employed in place of the spherical lens. However, it is a problem that the production of the cylindrical lens is more difficult than the spherical lens.